The present invention relates to a system for acoustically measuring the length of a pipe disposed under a road by using sound wave, and more particularly to a calibration system provided for calibrating change of speed of sound caused by pressure, temperature and component of gas in the pipe.
A pipe of a pipeline such as a gas main is disposed under a road for a long time so that the pipe may be corroded or broken by external force. In such a case, there is a method for lining an inner surface of a distributing pipe branched from the pipe-line with coating material such as resin. In order to repair the distributing pipe, there has been developed a method in which the inner surface of the pipe is repaired without digging the pipe.
In such a method, if the amount of resin exceeds a desired value, the resin is unnecessarily used. If the amount of resin is less than the desired value, the pipe is insufficiently lined. Accordingly, it is necessary to accurately measure the length of the distributing pipe to be repaired.
FIG. 3 shows a conventional measuring system for acoustically measuring the length of the distributing pipe. The measuring system is adapted to measure a distributing pipe 28 having a bent portion branched into a residential lot of a customer, from a gas main 29 which is laid under a street. One end of the distributing pipe 28 is connected to the gas main 29 by way of a joint. The other end of the pipe 28 is connected with a service riser, to an end of which a gas meter 15 is normally connected.
The measuring system comprises a pipe length measuring unit 21 and a cylindrical connecting member 16 for connecting the unit 21 to the pipe 28 through a lead 16b. The connecting member 16 is mounted on the end of the distributing pipe 28 by removing the gas meter 15 therefrom.
As shown in FIG. 4, the connecting member 16 has an inner threaded portion 16a formed at an end thereof to be engaged with an outer threaded portion formed on the end of the distributing pipe 28. On the other end of the connecting member 16, a sound emitter 25 and a sound receiver 26 are provided to be exposed to the pipe. A temperature sensor (thermistor) 27 is provided in the connecting member 16 adjacent to the sound emitter and sound receiver for measuring the temperature in the pipe.
The pipe length measuring unit 21 comprises a CPU 22, an amplifier 17 connected to the sound emitter 25, an amplifier 18 connected to the sound receiver 26, a D/A converter 19 connected to the amplifier 17 and to the CPU 22, an A/D converter 20 connected to the amplifier 18 and to the CPU, a memory 24 selectively connected to the CPU through a switch S, and a display 23 connected to the CPU. The temperature sensor 27 is connected to the memory 24.
In an measuring operation, the CPU 22 produces a command signal in the form of pulses for emitting a sound wave. The signal is applied to the D/A converter 19 to be converted into an analog signal. The analog signal is amplified by the amplifier 17. The sound emitter 25 is operated to emit a sound wave into the pipe 28. The sound wave is reflected at discontinuous positions such as joint, bent portion and opening portion. The reflected sound is received by the sound receiver 26 and amplified by the amplifier 18. The amplified signal is converted into a digital signal at the A/D converter 20 and applied to the CPU 22. The CPU measures a period of time from emitting of the sound in the pipe 28 to receiving the reflected sound in accordance with a time measuring circuit provided therein and calculates the length of the pipe 28 based on the measured period of time and the speed of the sound. The length of the pipe is displayed on the display 23.
The memory 24 stores data of the speed of the sound corresponding to the type of gases (such as air or natural gas) and the temperature detected by the sensor 27. The operator operates to change the switch S in dependency on the type of gas and the detected temperature so as to set a standard value of the speed of the sound.
However, the speed of sound largely changes in dependency on the temperature and component of gas in the pipe.
Therefore, for the pipeline of town gas, speeds of sound are previously measured at every one or a few grades of the temperature between 0 and 30 degrees under a constant pressure for every kind of gas. The data of the relationship between the speed of sound and the temperature and component of gas are stored in the memory 24. The switch S is operated for selecting data in accordance with conditions. Alternatively, an equation of the relationship is provided for correcting the measured value based on the temperature.
Accordingly, the handling of the system is complicated because of the measurement of temperature, the handling of the switch and others. Moreover, the length of the pipe can not be measured with accuracy because of measurement error.